I GIVE UP!! Figure this out if you dare!

[Guest]

Hi all,

This problem is absolutely maddening!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

My '80 550L has had a problem almost the entire 15 years I've owned it, and no matter what I do I haven't been able to solve it. Hopefully someone here has a solution once and for all ... here goes...

On moderate to hot days, if the bike is ridden at highway speeds (60mph+) for more than a few minutes, it will eventually start to bog down and slowly lose speed until finally it crawls to the side of the road (at full throttle) and dies... It will not restart until it has cooled off, anywhere from five to fifteen minutes, after which it will run somewhat normally as long as it is kept at slower speeds. If you run it back up to highway speeds, it will eventually fail again.

I initially thought that some part of the ignition system was overheating and failing, so last fall I replaced the ignition parts with a Dyna S, Accel coils, and Accel wires. All seemed to be fine until this week, when the bike was first subjected to extended running in warm weather. Same problem again.

I've never had charging problems so I didn't replace the stator or regulator, and when the bike dies there is plenty of cranking power available so I can't imagine that the charging system has anything to do with it.

I'm at a loss ... the only possible thing my buddy and I can come up with is that the rubber boots are perhaps cracked, overheating and expanding causing a fuel starved condition until they cool down and shrink again. The bike seems to run more or less normally at all other times.

Has anyone else ever experienced this sort of problem? Any ideas? It's driving me nuts!

Thanks in Advance!!!!!!!!
Steve 8)

[Anonymous]

Yes, it sounds like a combination problem. What is the max rpm get when you are running weak?

Also, can you tell which cylinders are firing when it is running weak? It might be helpful to know which you are losing?

Michael

[Planecrazy]

Good questions! I don't know the answers to either one, although when the bike is on the highway in sixth gear I estimate the RPM's are moderate to high ... as the bike begins to bog they steadily decrease and no amount of additional throttle holds speed for very long before it's wide open and still losing ground.

I'm not even sure how I would be able to determine which cylinders are shutting down when it occurs because, after all, I'm at highway speed with a helmet on trying to avoid becoming a non-moving target for other traffic... Unless someone comes up with another reasonable explanation, I'm going to have to assume that the boots (which are original) probably have cracks in them that I can't see. I'll have to get replacements and hope for the best.

Thanks,
Steve 8)

[Anonymous]

So if it's 30 degrees out, it'll run fine for hours on end at 60+? (Obviously, you'd be a popsicle, but that's beside the point, right?).
I'm sure you've tried taking the gas cap off, switching to prime, cleaning the petcock screen, checking the fuel line for kinks/blockage, etc....

[earlfor]

Steve, I will bet that when it is really cold out, you ride slower than you do when it is hot outside. Your fuel consumption rate is less at low speeds.
when you run at highways speeds, betcha you are using fuel faster than your tank vent can allow air in to displace fuel used, Engine has to pull fuel against a tank vacuum and you bog down and die when it reaches a point of no longer being able to. I think the fix for your bike is to buy a new gas cap. ROTFLMAO eheh hehe hehe e Ieeee ha ha :-)
Try a highway speed run with the cap loose on the tank so air can get in and lemme know what happens.

Earl

Good questions! I don't know the answers to either one, although when the bike is on the highway in sixth gear I estimate the RPM's are moderate to high ... as the bike begins to bog they steadily decrease and no amount of additional throttle holds speed for very long before it's wide open and still losing ground.

I'm not even sure how I would be able to determine which cylinders are shutting down when it occurs because, after all, I'm at highway speed with a helmet on trying to avoid becoming a non-moving target for other traffic... Unless someone comes up with another reasonable explanation, I'm going to have to assume that the boots (which are original) probably have cracks in them that I can't see. I'll have to get replacements and hope for the best.

Thanks,
Steve 8)

[Anonymous]

Your exact problem happened to me when I first got my bike, except that when that happened to mine, it was also blowing some bluish white smoke and dumping some oil out the vent hose from the air box (which comes from the top of the engine just above the cam chains - acting as though the crankcase was getting pressurized). This was in early March, but it was 50 or 60 degrees out, and it only happened if I was on the freeway for more than 10 minutes straight. Since I had just gotten it and didn't know when any maintenance had been performed, I changed spark plugs, points and condenser, and changed oil and filter, using Amsoil synthetic oil. Besides the typical effects of switching to Amsoil (better power, smoother running, less oil consumption) I could suddenly ride it as fast and as long as I wanted without the weird power loss problem.

I'm not sure exactly why my bike was doing it, or why yours is, and as mopolopo said, it sounds like a combination of things creating those specific symptoms. I just wanted to share that because I read your post and exclaimed "that's EXACTLY what mine did when I first got it!!" I'm sure you've changed your oil and spark plugs numerous times in the 15 years you've had the bike, so I don't have any recommendations for you on how to fix it other than trying synthetic oil - in my experience with many different cars and motorcycles, it never fails to make it run better, have more power, better fuel economy, etc, plus it sometimes solves weird problems like making my old Subaru stop knocking, making my Geo leak less oil, fixing my weird power problem on my GS, etc. Hope you get it figured out (after all these years)!

Alex

[first timer]

you can take the cap apart and clean it, go nuts with a tooth bruch and wd40. i would guess the cap vent is cloged or bad petcock, check the plugs too.

ryaN

[Anonymous]

Steve, I will bet that when it is really cold out, you ride slower than you do when it is hot outside. Your fuel consumption rate is less at low speeds.
when you run at highways speeds, betcha you are using fuel faster than your tank vent can allow air in to displace fuel used, Engine has to pull fuel against a tank vacuum and you bog down and die when it reaches a point of no longer being able to. I think the fix for your bike is to buy a new gas cap. ROTFLMAO eheh hehe hehe e Ieeee ha ha :-)
Try a highway speed run with the cap loose on the tank so air can get in and lemme know what happens.

I think Earl is right on. You're starving for gas. I had the same thing happen on my old Kaw, and it was actually the fuel filter. It could not keep up with the demand of the engine - after you'd let it set a while, the gas would flow through, fill up the carbs, and all was well again.

[Jeff Tate]

The same thing used to happen alot back in my dirt biking days. I saw alot of bikes up and quit because the cap vent was clogged with mud and crud. Let'em sit awhile and they'd run again... for awhile.

[Planecrazy]

You guys are AWESOME!! I swear I would never in a million years (or at least the last fifteen) even thought for a moment to suspect the gas cap!

I will definitely have my buddy (who is currently the "legal" owner of the bike) try it with the cap on loosely and see what happens.

Regarding the cold weather riding, I actually rode the bike into Chicago on December 9th for the Toys for Tots ride (extensive 60+ riding) and had no trouble whatsoever with this problem, so I'm not 100% sure the cap is the issue, but the cold weather may have caused the seal on the cap to shrink enough to prevent a vacuum situation from developing as easily.

I'll let you know what we find out, and thanks again for ALL the suggestions to date!!!

Steve 8)

[chuckycheese]

Me too

I also think the gas cap vent is almost surely the problem. The next time it quits on you, open the gas cap. I'll bet you'll hear a rush of air and that the bike will start immediately.

I had a Harley with exactly those symptoms and it was the gas cap. I also had a Triumph (car) that did the same thing. In both instances, it happened much more easily in hot weather (because pressure builds up much more quickly on a hot day). I think you've got an easy, cheap fix.

[Planecrazy]

Now if only the weekend weather would dry up so that my buddy could take the bike out and confirm the revelations you've all presented to us!!

Regards,
Steve 8)

[Anonymous]

Thought I'd also throw in my vote for the tank not venting properly.

[focus frenzy]

another vote for gas cap!
the seal will expand a slight amount with the warmer weather.

[HiSPL]

I vote for the gas cap too. However, if that turns out to not be the case then try replacing your vacuum line from the #2 carb to the petcock. I had one that would kink up when it got warm enough and shut off the vacuum supply to the petcock.

Good luck...